The present invention relates generally to an information signal transmitting method and an apparatus for carrying out the same as well as an information signal receiving method and an apparatus therefor. More particularly, the invention is concerned with methods and apparatus for transmission and reception of information signals which can profitably be used in information transmitting/receiving systems in which information signals are transmitted or received via a transmission path or channel which is rather poor in respect to the transmission quality and in which a high bit rate is difficult to realize, as exemplified by a frequency-modulated (FM) multiplex broadcast for mobile objects or mobile receiver stations.
In recent years, the frequency-modulated multiplex broadcast has widely been put to practical use with the aim of providing new services such as a sound broadcast separate from a stereophonic sound broadcast, a character information presentation and other services by making use of idle of empty spectral regions for multiplexing of digital signals in an FM stereophonic broadcast.
One example of a frequency-modulated multiplex broadcast for the mobile objects or receiver stations, is the RDS system recommended in "CCIR RECOMMENDATION 643". In this system, a (26, 16) code which is rather poor in error correcting capability as the error correction code is employed. Under the circumstances, information signal reception is performed by resorting to a request repeat system for data exchange instead of error correcting reception scheme in the present state of the art.
An example of a fixed receiver station system, is FM multiplex broadcast in Japan, in which a frame structure of 272.times.34 bits is employed together with a (272, 190) code for error correction, as is illustrated in FIG. 12 of the accompanying drawings. More specifically, referring to FIG. 12, this frame is composed of a frame code (FC) of 18 bits, a mode control code (MC) of 16 bits, and a data field composed of 34 packets each of which includes a data block of 190 bits and a parity block of 82 bits so that data of 272.times.34 bits in total can be transmitted with one frame. In this frame structure, the (272, 190) code is used as the intra-frame error correcting code.
In the FM multiplex broadcast using the frame of the structure described above, the information signals are sent out in a sequence traversing the frame obliquely, as is indicated by arrows 101 in FIG. 12. Consequently, the receiver is required to be equipped with a frame memory, wherein decoding of the error correcting code is performed after having received one frame wholly.
Further, a data transmission using the (272, 190) code is adopted in the character information broadcast service in Japan, as is disclosed, for example, in U.S. Pat. No. 4,630,271 corresponding to Japanese Patent Applications Nos. 6579/1983 and 54002/1983.
Of the hitherto known systems mentioned above, the system in which the (26, 16) code is employed is poor in the error correction capability is not suited for the transmission of data such as those of idiographic characters typified by Japanese or Chinese language, traffic information or the like which is very susceptible to the influence of the error.
On the other hand, with the frame structure of 272.times.34 bits with the (272, 190) error correcting code adopted in the fixed receiver station system, the number of times the frame synchronizing signal is sent out is small, involving possibly dislocation in the frame synchronism in the transmission path which suffers not a few errors. Although the oblique sending-out sequence is advantageous in that errors are dispersed, there arises such a disadvantage that excessively large amounts of time is taken for making available the received data by decoding in the case of the mobile receiver station system whose bit rate is low when compared with the fixed receiver station system. Thus, the frame structure is not suited for transmission and reception of such information which must be made available as soon as possible after turn-on of a power supply. As the information of this type, there may be mentioned, for example, information required for displaying the name or identifier of the broadcasting or transmitter station. Besides, it is noted that the effect of the interleave can not be expected to any satisfactory extent with the number of blocks on the order of 34 or so because of burst error due to the fading phenomenon encountered in the mobile receiver system, to another disadvantage. Additionally, in the case of the FM multiplex broadcast of the fixed receiver station system, data decoding can not be performed before one frame has completely been received by the receiver, giving rise to a problem that a remarkable time delay intervenes between the data reception and the data decoding. Accordingly, when this receiver is employed in the mobile receiver system, the information required immediately such as station identifying information can not be made available instantly upon power-on of the receiver or upon switching of channels, presenting a problem in practical applications because the bit rate in the mobile receiver station system is low as compared with that of the fixed receiver station system, as mentioned previously. Furthermore, it should also be pointed out that in such extremely unfavorable receiving conditions which preclude the complete reception of one frame, the received data cannot be utilized at all, to a further disadvantage.